1. Technical Field
Example embodiments of the present invention relate to a microporous polyolefin film suitable as a separator for batteries and thermal properties thereof. A microporous polyolefin film according to the present invention has a film thickness of 5-40 μm, a porosity of 30%-60%, a permeability of 2.0×10−5-8.0×10−5 Darcy, a maximum pore size determined by the bubble point method of not more than 0.1 μm, a puncture strength of 0.20 N/μm or more at room temperature and 0.05 N/μm or more at 120° C., and a maximum shrinkage ratio in the transverse direction (TD) when subjected to a thickness-normalized external force in TMA (thermo-mechanical analysis) of not more than 0%.
2. Description of the Related Art
Microporous polyolefin film is in wide use for separators for batteries, separation filters, membranes for microfiltration, and the like due to its superior chemical stability and physical properties. Of these, separators for secondary batteries require the highest quality along with the battery safety requirement. Recently, as capacity and output of secondary batteries are improved, requirements for separators with respect to thermal stability of separators and electrical safety of secondary batteries during charge and discharge are becoming stricter. In the case of lithium secondary batteries, poor thermal stability of separators may lead to damage of the separators caused by temperature increase inside the batteries or short circuit between electrodes resulting therefrom. As a result, there is a risk of overheating of the batteries or fire. Further, as the application of the secondary batteries is extended to hybrid cars and other fields, safety of the batteries against overcharge has become an important requirement. Therefore, separators are required to endure the electric voltage caused by overcharge.
The factors affecting the thermal stability of a battery include shut down temperature, melt down temperature and melting shrinkage ratio in the transverse direction (i.e., a direction perpendicular to the winding of electrode/separator) of the separator, strength of the separator at high temperature, and the like.
Shutdown temperature is the temperature at which the micropores of the separator are closed to shut the electric current when the inside of the battery is abnormally overheated. Melt down temperature is the temperature at which the separator is subjected to melting and the electric current flows again (i.e., the temperature at which short circuit occurs between electrodes) when the battery temperature increases beyond the shutdown temperature. To ensure thermal stability of a battery, it is preferred that the shutdown temperature is low and the melt fracture temperature is high.
Melting shrinkage ratio in the transverse direction refers to the degree of shrinkage occurring when the separator is melted. If the melting shrinkage ratio in the transverse direction is large, the edge of the electrodes is exposed as the separator of the battery shrinks at high temperature, resulting in short circuit between electrodes and overheating, fire, explosion, or the like. Even if the separator has a high melt down temperature, a high melting shrinkage ratio in the transverse direction may lead to short circuit between electrodes as the edge of the electrodes is exposed while the separator is melted.
A high strength of the separator at high temperature is required to prevent damage of the separator at high temperature which may result from dendrites produced at the electrodes during charge and discharge of the battery and to, thereby, prevent short circuit between electrodes. A weak strength of the separator at high temperature may lead to short circuit caused by the fracture of the separator. In this case, overheating, fire, explosion, or the like may occur due to short circuit between electrodes.
Therefore, among the above-mentioned requirements of the separator for thermal stability of the battery, the melting shrinkage ratio in the transverse direction and the strength of the separator at high temperature are the most important because they prevent short circuit between electrodes fundamentally.
The approaches of improving thermal stability of a battery by increasing the melt down temperature of the separator includes crosslinking the separator, adding an inorganic material, using a heat-resistant resin, or the like.
The method of crosslinking the separator is disclosed in U.S. Pat. Nos. 6,127,438 and 6,562,519. According to these patents, crosslinking of film is carried out using electron beam irradiation or chemically. However, crosslinking using electron beam irradiation is disadvantageous in that an electron beam irradiation apparatus is required, production rate is limited, and quality unevenness may occur due to nonuniform crosslinking. And, chemical crosslinking is disadvantageous in that the process of extruding and mixing is complicated, it is highly probable that film gelation may occur due to nonuniform crosslinking, and a long time of high-temperature aging is necessary.
U.S. Pat. No. 6,949,315 discloses a method blending UHMW (ultra high molecular weight) polyethylene with 5-15 weight % of an inorganic material such as titanium oxide to improve thermal stability of the separator. However, this method is disadvantageous in that the use of UHMW polyolefin increases load of extrusion reduces mixing performance, and reduces productivity due to insufficient stretching. Further, the addition of inorganic material may lead to poor mixing and nonuniform quality and generating pinholes resulting therefrom, or poor film properties because of lack of compatibility at the interface between the inorganic material and the polymer resin.
U.S. Pat. No. 5,641,565 discloses a method of blending a resin having superior heat resistance. This method requires UHMW polyethylene having a molecular weight of 1,000,000 or higher in order to prevent deterioration of physical properties caused by the addition of the different resins like polypropylene and inorganic material. Further, the overall process is complicated because additional processes are required for extraction and removal of the inorganic material.
In addition to the above-described problems, the aforesaid methods only aim at improving the melt down temperature of the separator, and do not consider the melting shrinkage in the transverse direction or the strength of the separator at high temperature at all. As a consequence, they are limited in improving the thermal stability of batteries and are not widely utilized for commercial purposes.
A method of reducing shrinkage of the separator in the transverse direction is presented in Japanese Patent Laid-Open No. 1999-322989. In this method, the film is stretched only in the longitudinal direction or the total stretch ratio is reduced to reduce the thermal shrinkage in the transverse direction. Accordingly, superior physical properties cannot be attained because the improvement of physical properties achieved through stretching cannot be expected. The products described in the examples have a very low puncture strength at room temperature at about 0.06-0.11 N/μm. Although physical properties at high temperature are not mentioned, improvement of thermal stability of battery at high temperature may not be expected considering that puncture strength tends to decrease as temperature increases.
Although Japanese Patent Publication No. 2003-119306 discloses a separator having a shrinkage ratio less than 1%, separator strength was not measured at all and, in particular, strength at high temperature is not considered at all. Therefore, maximizing battery safety at high temperature may be difficult to be expected.
Another important factor with respect to the safety of a secondary battery is the battery overcharge characteristics. Overcharge characteristics refer to safety against leakage of electrolyte, explosion, fire, or the like when the battery is overcharged. In the current situation where the utilization of high-capacity and high-power batteries is in the increase, for example, in hybrid cars, they are one of important requirements for the batteries. With respect to the improvement of overcharge characteristics of a secondary battery, the presence of excessively large pores in the separator is undesired. In general a large pore is advantageous with respect to the improvement of battery life time and output. However, when the pore size is above a predetermined value, battery safety such as overcharge characteristics may be impaired without further improvement of battery life time or output. The pores inevitably have a size distribution during their formation. If excessively large pores are formed in the separator, they may impair overcharge characteristics of the battery because they provide weak resistance to the electric voltage applied during charging of the battery.
Korean Patent Publication No. 2006-0103932 discloses a separator with a narrow pore size distribution for high voltage resistance and superior shut down performance. However, this patent does not consider the battery safety problem such as overcharge characteristics that may occur when the pore size is excessively large. The reason why the ratio of average pore size to maximum pore size (i.e., pore size distribution) is important is because, as mentioned in the patent, the maximum pore size is an important factor with respect to battery safety such as voltage resistance.
As described above, a separator having low melting shrinkage ratio in the transverse direction as well as high puncture strength at high temperature, which are essential factors required for thermal stability of high-capacity, and high-power secondary batteries, and having an adequate pore size for safety during charge and discharge has not been developed in the related art.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.